1. Field of the Invention
This invention relates to a method for the fabrication of a Josephson junction.
2. Prior Art
In recent years, it has been necessary to deal with a great quantity of information and to speedily deal with highly technological calculations. For this reason, computers have now been extensively used to process such data. It has now been desired to develop high-speed switching elements to enable computers to process data more rapidly. One of such switching elements is a Josephson junction, and among Josephson junctions, a Josephson tunnel junction is most preferred in that it has excellent switching characteristics. The Josephson tunnel junction is known as an ultra-high speed switching element. The Josephson tunnel junction can be operated at a lower rate of power consumption and at an extremely higher rate of speed in comparison with silicon semiconductor devices commonly used in computers as a switching element.
The Josephson tunnel junction is of a simple construction and comprises a base electrode, a counter electrode, and a layer of insulating barrier sandwiched between the base and counter electrodes, the barrier layer having a thickness of 30 to 70 Angstroms.
A typical example of Josephson junctions employs either lead or lead alloy as a base material, and an investigation of integrated junctions has now been made using such a base material. The Josephson junction made of lead or lead alloy has the advantage that it has good reproducible characteristics. However, since lead alloy has a relatively low melting point and is soft, its junction characteristics are liable to be degraded by a temperature difference or thermal cycling between an operating temperature (4.2 K in liquid helium) and room temperature. In addition, a fine processing or working of lead alloy is rather difficult because of its soft nature. On the other hand, superconductors made of either niobium (Nb) or niobium compounds have a high melting point and are hard. Therefore, such a superconductor has good anti-thermal cycling properties and can be finely processed or worked. A problem of this superconductor is that it has not yet had junction characteristics suitable for an integrated junction structure. Accordingly, a study of junction fabrication techniques with respect to superconductors of niobium or niobium compounds has now been made. "Niobium oxide-barrier tunnel junction" is disclosed by Ronald F. Broom et al in IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-27, OCTOBER 1980, PAGES 1198-2007. They describe the fabrication and electrical characteristics of niobium oxide-barrier tunnel junctions. Also, the oxidation of lead films by rf sputter etching in oxygen plasma is disclosed by J. H. Greiner in Journal of Applied Physics, Vol. 45, No. 1, January 1974, Pages 32-37. A method of forming thin oxide layers by sputter etching in a low-power rf oxygen discharge (rf oxidation) is described in it.